LEL (lower explosive limit) control with automatic calibration capability

ABSTRACT

The LEL (Lower Explosive Limit) control includes a gas analyzer normally connected to receive sample gas from the evaporation enclosure of a press dryer section and which, during calibration, is sequentially connected to first analyze solvent free gas and then gas having a known solvent content. Situated with the analyzer is a detector the resistance of which changes in proportion to the temperature within the flame cell of the analyzer. The detector forms a portion of a bridge circuit, which also includes a potentiometer. The output of the bridge is amplified, compared to a selected reference voltage and a signal representing the difference therebetween is processed. The processed signal is combined with a signal representing the web speed to form a resultant signal to control a pneumatically operated damper in the enclosure exhaust conduit. Automatic calibration of the system is initiated periodically and takes place in two phases. During the first phase of calibration, the potentiometer is set in accordance with the difference between the amplifier output derived from analysis of the solvent free gas and a 0% reference voltage. During the second phase of calibration, the gain of the bridge output amplifier is set in accordance with the difference between the amplifier output derived from the analysis of the known solvent concentration gas and a full scale reference voltage. When calibration is taking place, the output of the comparison circuit is inhibited and the damper position is determined in accordance with the web speed alone. Emergency stop of the press occurs when the output of the amplifier exceeds a given level.

The present invention relates to the LEL (lower explosive limit)controls and more particularly to an LEL control having automaticperiodic calibration capability.

LEL controls of a variety of different structures are well known in theart. Such controls are commonly utilized as energy saving devices and asfailsafe mechanisms in the dryer sections of printing presses or thelike and, therefore, the present invention will be described in thiscontext.

In a printing press, after the ink solution has been deposited on theweb, the web is passed through a dryer. The dryer includes an enclosurewherein heated air is passed over the web to evaporate the solvent fromthe deposited ink solution. The evaporated ink solvent and air forms apotentially explosive mixture if the solvent concentration thereof isgreater than a given level, commonly referred to as the lower explosivelimit (LEL). In order to insure that the air within the evaporationenclosure does not contain a solvent concentration greater than thelower explosive limit and, therefore constitutes a safety hazard, it ispossible to continuously withdraw all of the solvent laden air from theenclosure connected thereto. The solvent laden air is then transferredto a pollution control device which processes the exhaust prior toreleasing same into the atmosphere.

The amount of energy required to operate the pollution control apparatusis proportional to the volume of exhaust which must be processed. If anLEL control is utilized to regulate the position of a damper located inthe exhaust conduit, it is possible to accurately regulate the solventconcentration in the evaporation enclosure and therefore prevent thesolvent concentration from exceeding a safe level. This may be achievedby recycling a regulated proportion of the solvent laden air to theinput side of the enclosure and by controlling the exhaust damper suchthat only a relatively small proportion of the solvent laden air istransferred to the pollution control apparatus. This methodsubstantially reduces the amount of energy which is required to operatethe pollution control apparatus because the amount of exhaust to beprocessed is substantially reduced. Thus, when used in this manner, theLEL control acts as an energy saving device.

In addition, the LEL control can be connected in a failsafe manner suchthat if the solvent level concentration within the evaporation enclosurerises above the lower explosive limit, this condition can be detected,an "emergency stop" signal generated and the press can be shutdownautomatically. This will prevent further build up of the solventconcentration, possibly leading to a hazardous condition and will insurethat any malfunction in the system will be corrected before an explosiontakes place.

Because of the critical nature of the above-described functions, it isnecessary that the LEL control operate accurately. In order to insureaccuracy, calibration of the control is required on a frequent basis.Obviously, during the calibration period, which requires severalminutes, the LEL control can not function to regulate the exhaust damperin accordance with the sensed solvent concentration level. Therefore, inprior art systems, calibration must take place when the press is notoperating or the press must operate without control of the solventconcentration level during calibration.

Prior art systems regulate the position of the exhaust damper inaccordance with the solvent concentration alone. Thus, in the event thatthe solvent concentration monitoring portion of the system fails, thepress must be shut down or run in an uncontrolled manner. In order toavoid this problem, it would be advantageous to design a system whichwill operate in a "back up" mode which will take into account the worstcase conditions and continue to regulate the solvent concentration at anet energy savings. This can be accomplished by positioning the exhaustdamper in accordance with a signal proportional to the web speed. Thepress will therefore continue to run unless a dangerous solventconcentration occurs or the exhaust damper cannot respond to the controlsignal.

The intrinsic safety of LEL control is a matter of concern because ofthe environment in which the control operates. Some of the components,such as the reference gas sources and fuel sources for the gas analyzerand power supply can be located in a safe area. However, certaincomponents must be located in the hazardous area. It is necessary todesign the components located in the hazardous area in such a manner soas to prevent an explosion from occuring.

It is, therefore, a prime object of the present invention to provide anLEL contro 1 having automatic calibration capablity wherein thecalibration cycle is automatically periodically initiated and duringcalibration the LEL level continues to be controlled, such that the safeoperation of the press is uninterrupted.

A further object of the present invention is to provide an LEL controlhaving automatic calibration capability wherein during the calibrationcycle the system regulates the exhaust damper position in accordancewith the speed of the web alone.

It is another object of the present invention to provide an LEL controlhaving automatic calibration capability wherein an "emergency stop"signal is automtically generated in the event that the solventconcentration level exceeds a present value.

It is still further object of the present invention to provide an LELcontrol having automatic calibration capability wherein calibrationtakes place at both the zero solvent concentration level and at the fullscale solvent concentration level.

It is still another object of the present invention to provide an LELcontrol having automatic calibration capability which will operate in anintrinsically safe manner.

In accordance with the present invention, the LEL control regulates theposition of an exhaust damper associated with an evaporation enclosurethrough which a solvent laden web passes. The control includes meansoperably connected to the enclosure for sensing the solventconcentration therein and for generating a first signal proportional tothe sensed solvent concentration. Means operably associated with the webare provided for sensing the web speed and for generating a secondsignal proportional thereto. The control means includes means forcombining the first signal and the second signal to form a controlsignal. Also included are means for calibrating the first signalgenerating means, the calibrating means comprising means for inhibitingthe output of the first signal generating means during calibration.Means are provided for regulating the position of the damper inaccordance with the control signal.

The first signal generating means comprises a gas analyzer which isnormally connected to receive sample gas from the dryer enclosure. Thegas analyzer is also connected to a first source of solvent free gas anda second source of a gas of known solvent concentration. During thefirst phase of calibration, first valve means connect the first gassource to the chamber and during the second phase of calibration, secondvalve means connect the second gas source to the chamber, such thatcalibration at 0% LEL and full scale LEL can be achieved.

The first signal generating means further comprises a resistance bridgeand an amplification means connected to the output thereof. The bridgeincludes a variable resistance means, located in the flame cell to sensethe temperature thereof and a potentiometer. Processing means areprovided for processing the first signal including comparison means forcomparing the amplifier output with a selected one of three voltagesrepresenting, respectively, a set LEL level, 0% LEL level and full scaleLEL level. The output of the comparison means is partially processed andconnected to an analog switch which serves to inhibit the output of theprocessing circuitry during calibration. When the system is not beingcalibrated, the output of the processing circuitry is combined with thesecond signal based on the web speed, to form the control signal. Duringcalibration, the control signal is based on the second signal alone. Thecontrol signal is utlized to position a damper located in the exhaustconduit connected to the enclosure.

The system normally operates in the LEL mode wherein gas from theenclosure is analyzed and the output of the amplifier is a function ofthe solvent concentration in the enclosure. This output is compared to aset LEL voltage and a signal representing the difference therebetween isprocessed, the web speed signal added thereto and the result is used tocontrol the position of the damper.

Calibration is automatically initiated by a timing circuit and takesplace in two phases. During the first phase, the flame cell is connectedto receive the solvent free gas, which is then analyzed. The output ofthe amplifier is compared to a 0% LEL reference voltage and the signalrepresentation of the difference therebetween is utilized to adjust theresistance bridge for the correct zero setting. During the second phaseof calibration, the gas of known solvent concentration is analyzed. Theoutput of the amplifier is compared to a preset reference voltage andthe signal representation of the difference therebetween is utilized toadjust the gain of the amplifier to the correct full scale setting.

While calibration is taking place, the analog switch inhibits the outputof the first control signal processing circuitry and the damper ispositional in accordance with the web speed signal alone. The web speedis derived from the output of a tachometer connected to the web.

The LEL control also includes means for monitoring the first signal andfor generating an "emergency stop" signal when the first signal exceedsa given value. This signal is utilized to prevent further solventaccumulation by shutting down the press.

Preferably, the control means also comprises manually actuatable meansto disable the first control signal generating means. Thus, it ispossible to manually actuate the LEL control to regulate the damperposition in accordance with the web speed alone.

In the event of a failure in the monitoring portion of the system, thecontrol will regulate the solvent concentration in accordance with theweb speed alone such that the press operatin will not be interrupted.The operation of the system in this mode still constitutes a net energysavings. An emergency stop will occur only for a dangerous solventconcentration in the dryer or if the exhaust damper cannot respond toits control signal.

The intrinsic safety of the control of the present invention is insuredby locating the source of known solvent concentration gas, fuel supplyfor the gas analyzer and power supply assembly in a non-hazardous area.In the hazardous area, the components are designed such that a faultcondition cannot produce a hot enough spark to ignite a solvent samplein its most easily ignitable mixture. The 115 VAC line voltage isisolated from the connections to the hazardous area. This isaccomplished by selecting a transformer whose primary and secondary areseparated by an insulating barrier, grounding its core and properlyfusing its input. Further isolation is accomplished by having a groundplane separating all primary from all secondary voltages. The secondaryvoltage is then limited by the wire wound barrier resistors which willfail open for a fault condition. In addition, the relays, beinginductive, are protected by redundant shunt diodes to dissipate theinductive kick when the relay is de-energized.

The gas analyzer itself is proected by enclosing all electricalcomponents in explosion proof enclosures. The web tachometer input isisolated by passing same through a zener barrier. The motorizedpotentiometers, and the electrical to pressure transducer coil, areshunted with redundant resistors. All connecting cables are adequatelyseparated and provided with safety shielding. In this manner, thecontrol is designed in an intrisically safe manner.

To the accomplishment of the above and to such other objectives that mayhereinafter appear, the present invention relates to an LEL control withautomatic calibration capability, as described in the presentspecification and recited in the annexed claims, taken together with theaccmpanying drawings where like numerals refer to like parts and inwhich:

FIG. 1 is a schematic representation of the dryer section of a printingpress showing the manner in which the LEL control of the presentinvention is connected thereto;

FIG. 2 is a schematic diagram of the LEL Control of the presentinvention;

FIG. 3 is a flow-diagram of the gas analyzer which is associated withthe control of the present invention;

FIG. 4 is a detailed block diagram of the control circuit of the presentinvention.

FIG. 5 is a schematic diagram of the resistnce bridge and signalamplification means of the present invention;

FIG. 6 is a schematic representation of the comparison means, analogswitch circuit and the zero crossing detector circuits of the presentinvention;

FIG. 7 is a schematic diagram of the signal processing circuitry and ablock diagram of the damper control devices of the present invention;

FIG. 8 is a schematic diagram of the drive circuits for the motorizedpotentiometers used to calibrate the system of the present invention;

FIG. 9 is a schematic diagram of the driver circuits for the warning,danger and flame out lamps of the present invention.

FIG. 10 is a schematic diagram of the mode selector switch, speed modeenable circuit, LEL lamp driver circuit and power indicator lamp drivercircuits of the present invention;

FIG. 11 is a schematic diagram of the calibration trigger circuit andthe zero solvent concentration level gas relay control circuit of thepresent invention;

FIG. 12 is a schematic diagram of the known solvent concentration levelgas relay control circuit and the propane driver and timer circuit ofthe present invention;

FIG. 13 is a schematic diagram of a power lamp indicator driver circuitand of the first potentiometer travel limit detector of the presentinvention;

FIG. 14 is a schematic diagram of the second potentiometer travel limitdetector circuit, the low-flow detector circuit and the calibrationfault indicator driver circuit of the present invention;

FIG. 15 is a schematic diagram of the emergency stop driver circuit andspeed lamp driver circuit of the present invention; and

FIG. 16 is a schematic diagram of the calibration Green lamp indicatorcircuit, purge delay circuit, test/sample selector driver circuit andair/methane selector drive circuit of the present invention.

As shown in FIG. 1, the LEL Control of the present invention isassociated with a gas analyzer, generally designated A, which isconnected to the control assembly, generally designated B. The gasanalyzer A and the control assembly B are shown as connected to a dryersection, generally designated C, of a printing press or the like. Dryersection C includes an evaporation enclosure 10 which is located directlyabove an inking station 12. As the web 14, which is to be imprinted,enters the inking station 12, it passes a tachometer 16, which can beany one of a variety of known commercially available tachometersdesigned for this purpose. Tachometer 16 measures the web speed andgenerates a signal proportional thereto. The web travels between idlerrollers 18 and around idler rollers 20. Thereafter, the web passesbetween an inking roller 22 and a pressure roller 24. Inking roller 22is partially immersed in an ink bath 25 and is provided with a pluralityof indentations on the surface thereof which, after passage through inkbath 25, retain small amounts of the ink solution thereon. As the web 14passes between inking roller 22 and pressure roller 24, the ink solutionsituated in the indentations on the surface of inking roller 22 istransferred to the surface of the web 14. Web 14 then travels intoenclosure 10 and around idler rollers 26. After completing the path oftravel through enclosure 10, the web exits the other side thereof and istransferred to the next printing section.

Adjacent to idler rollers 26, but spaced therefrom to permit web 14 topass therebetween, is a duct 28 having a plurality of outlet openingssituated in close proximity to the suface of web 14. Duct 28 is fed froma heater/blower apparatus 30. The input side of heater/blower apparatus30 is connected to a fresh air inlet conduit 32 and a recirculationconduit 34. Recirculation conduit 34 originates at the top of enclosure10 and the amount of solvent laden air which is recirculated fromenclosure 10 to heater/blower apparatus 30 is regulated by arecirculation damper 36 located near the entrance of the recirculationconduit 34. Originating also at the top of enclosure 10 is an exhaustconduit 38. The amount of exhaust which passes through exhaust conduit38, and thereafter to the pollution control apparatus (not shown), isregulated by an exhaust damper 40.

Enclosure 10 defines an enclosed area and the amount of exhaust which isdrawn through exhaust conduit 38 always approximtely equals the amountof fresh air which is drawn through fresh air inlet 32. Exhaust damper40 and recirculation damper 36 are always driven oppositely, that is, asexhaust damper 40 is opened, so as to permit more air into exhaustconduit 38, recirculation damper 36 is closed, so as to permit less airinto recirculation conduit 34.

Dampers 36 and 40 are pneumatically controlled in accordance with anelectrical control signal generated by control assembly B. As isexplained in detail below, during normal operation the control signal isgenerated by control assembly B in accordance with the web speed, assensed by tachometer 16, and in accordance with the solventconcentration within enclosure 10, as sensed by a gas analyzer A. Gasanalyzer A continuously samples the solvent concentration of the airwithin enclosure 10 through conduit 42. During calibration, when thecontrol signal is a function of the web speed alone, gas analyzer A willfirst test a sample having zero solvent concentration and thereaftertest a sample having a known solvent concentration (methane). Inputsfrom souces of each of these gases are provided and designated as 44 and46 respectively. An input from a source of fuel (propane) for the flamein the gas analyzer is provided by means of conduit 48. Further,compressed air, used to draw exhaust from the flame cell, is providedthrough conduit 50.

With reference to FIG. 2, this figure shows gas analyzer A connected toenclosure 10 by means of conduit 42 to obtain a sample from the dryer,to a souce of zero concentration gas (air) by a means of conduit 44, toa source 52 of gas of a known solvent concentration (methane in air), toa fuel supply such as a propane source by means of conduit 48 and to asource of compressed air by means of conduit 50. Gas analyzer A is alsoconnected to a power supply 52. Control assembly B, includes powersupply 52 and control circuitry 54, described in detail below, whichgenerates the damper control signal. The control signal, in accordancewith the setting of an exhaust damper position potentiometer 56, drivesan electric to pneumatic transducer 58, which is supplied withcompressed air from a pneumatic supply, through a conduit 60. Transducer58 drives recirculation damper pneumatic actuator 62 and exhaust damperpneumatic actuator 64. Inputs to control circuitry 54 include the outputfrom the gas analyzer A, and the output from web tachometer 16, as wellas the output from power supply 52.

FIG. 3 is a schematic diagram of the gas analyzer which forms a portionof the present invention. Gas analyzers are well known in the art in avariety of different forms. The analyzer described herein is model A1FFAFlammable Gas Detection System which is manufactured by ControlInstruments Corporation, North Caldwell, N.J. However, different gasanalyzers could be used for this purpose and the particular structure ofthe gas analyzer described herein should not be construed as alimitation on the present invention.

As shown in FIG. 3, compressed air (15 psi) enters the analyzer throughconduit 50, which is connected to an aspirator 66 through a valve 68.The aspirator provides suction at the exhaust side of the flame cell 70so as to draw exhaust fumes through exhaust conduit 72. A sample adjustvalve 74 is connected in exhaust conduit 72 in order to regulate theflow through the cell. A filter 76 is provided in exhaust line 72 toprotect aspirator 66.

Propane, which is utilized as fuel for the flame within cell 70, entersthe system through conduit 48. Connected to conduit 48 is a fuel filter78, a flow-control regulator 80 and a heated capillary (not shown). Theflow-control regulator 80 is controlled by a feedback signal monitoringthe flow (vacuum) in the flame cell, the feedback line being designated82.

Three gas inputs are available to the flame cell for analysis: air (zerosolvent concentration); methane (2.5% methane in air, a certifiedstandard mixture) and sample (taken from the evaporation enclosure)through conduits 44, 52 and 42 respectively. Each of these sources isselected, one at a time, by means of two three-way solenoid valves 84and 86. Valve 84 has conduits 44 and 52 as inputs (the former beingnormally opened and the latter being normally closed) and a connectingoutput conduit 88 which connects valve 84 to an input (normally closed)of valve 86. The other input (normally opened) of valve 86 is connectedto conduit 42 and the output thereof is connected to the input side ofthe flame cell 70.

Located above the pilot flame in the flame cell 70 (but not shown onthis drawing) is a resistance temperature detector, preferably comprisedof platinum wire sensor which changes resistance with changes intemperatures. Since solvents have fuel value, as they pass through thepilot flame and oxidize, heat is released. The amount of heat releasedis proportional to the solvent concentration of the gas and this issensed by the detector.

In order to calibrate the system of the present invention, zero solventconcentration gas (air) is first selected by means of the inputsolenoids 84 and 86 and analyzed. The output of the detector is used tobalance a resistance bridge located in the control means, of which thedetector forms a part, to 0% LEL setting. Then, the gas of known solventconcentration (methane) is selected and analyzed. This mixturecorresponds to 64% LEL and the electronics are then balanced to thisknown input. Since 40% LEL is a maximum operating point, 64% representsa full scale calibration. After the electronics are calibrated, thesample input (from the evaporation enclosure) is again selected andanalyzed. It is now possible for an accurate measure of the LELconcentration of the sample gas to be achieved.

Trim valves 90 and 92 are provided to balance the gas input flow ratesin conduits 44 and 52 respectively. A low flow switch 96 is provided inconduit 72 and connected to the control assembly. If the flow ratebecomes too low, the measuring accuracy of the analyzer will becomprimised and the output of the detector will be neglected through theactuation of low flow switch 96.

FIG. 4 is a detailed block diagram of the control assembly of thepresent invention. In this drawing, block 100 represents the bridgecircuit wherein the resistance of the detector, which is physicallylocated in the gas analyzer, is measured. This block also includes anamplification means to amplify the bridge output and motorizedpotentiometers which calibrate the bridge and adjust the gain of theamplifier.

The output of block 100, which represents the measured LEL voltage, isconnected to block 102 which includes voltage comparison means in theform of a differential amplifier. The differential amplifier makes threesets of comparisons: the first comparison is of the set LEL voltage tothe measured LEL voltage of the sample gas; the second comparison is ofzero volts to the LEL voltage of the zero solvent concentration gas forzero level calibration, and the third comparison is of a preset voltageto the LEL voltage of the known concentration gas for the full scalecalibration. The output of the differential amplifier means is alsoutilized to operate the motorized potentiometers in order to calibratethe bridge and amplifier means of block 100.

The output of the comparison means of block 102 is connected to block104. Block 104 contains electronics to partially process the amplifieroutput. Block 104 also contains switch means which are used to inhibitthe output of the processing circuitry under certain conditions, such asduring calibration, when the system is operating in the SPEED mode andthe resultant control signal is based on the second control signalalone, which is derived from the output of tachometer 16.

The output of block 104 is connected to block 106 which contains furthersignal processing circuitry. Block 106 receives the web speed signalfrom tachometer 16, the web speed signal being combined in block 106with the output of block 104. Block 106 also contains a level detectorto signal an emergency stop if the feedback signal from the exhaustdamper is invalidated. The output of block 106 is the damper controlsignal and is connected to an electric to pneumatic transducer 58 (shownin FIG. 2 but not shown in FIG. 4) which drives an exhaust damperposition actuator which in turn drives the exhaust damper and therecirculation damper.

It can thus be seen that during normal operation (LEL mode), the solventconcentration of the gas sample from the evaporation enclosure ismeasured and amplified in block 100 and compared to a set LEL voltage bymeans of a differential amplifier in block 102. The output of block 102is partially processed in block 104 and further processed in block 106where it is combined with the signal based on the web speed. The outputof block 106 drives an electric to pneumatic transducer which in turnpositions the dampers. In this matter, the exhaust damper is positionedin accordance with the solvent concentration level of the sample fromthe enclosure and the web speed.

During calibration or manual actuation of the SPEED mode, the switchmeans located in block 104 inhibits the voltage output of the block 102and the resultant control signal is proportional to the input from webtachometer 16 alone. When calibration is initiated, the SPEED mode isautomatically selected and the exhaust damper is no longer poistionedwith respect to the output from the bridge circuit but is insteadpositioned only in accordance the second control signal based on theoutput from web tachometer 16, which is proportional to the web speed.This permits the bridge in block 100 to be calibrated without adverselyaffecting the damper position.

Calibration takes place in two phases. First, a calibration at the 0%solvent concentration level takes place. By means of a relay, a switchin block 102 is actuated so as to cause the differential amplifiertherein to make a comparison of zero volts to the amplified bridgeoutput, when the zero solvent concentration level gas (air) is analyzed.Block 110, which is connected to the output of the differentialamplifier in block 102 by a relay, during this phase of calibration,contains the bridge calibration potentiometer drive circuitry whichdrives potentiometer MP1 in order to calibrate the bridge at the 0% LELsetting. Block 108, also connected to receive the output from block 102during this phase of calibration, contains a zero crossing detectorwhose output Z_(o) prevents motorized potentiometer MP1 from drivingpast the calibration point.

After the 0% solvent concentration level setting has been calibrated,the gas of known solvent concentration is a analyzed, the output of theamplifier compared to the full scale reference voltage and thecomparison means output is fed to block 114 by a relay. Block 114, whichcontains the amplifier gain potentiometer drive circuitry, willcalibrate the gain of the bridge output amplifier, located in block 100,to the full scale setting. Block 112, which contains a zero crossingdetector, also receives the output of the comparison means during thisphase of calibration and generates a signal S_(o) to preventpotentiometer MP2 from driving past the calibration point.

The calibration cycle is initiated periodically by block 116 whichtriggers the cycle. The output of block 116 is transferred to blcok 118which includes the 0% solvent concentration gas relay control. One ofthe outputs of block 118 is connected to block 122 which controlssolenoid valve 86 in the gas analyzer such that the B 0% solvent levelconcentration gas (air) is analyzed. Another output Z of block 118 isconnected to the bridge calibration potentiometer circuitry in block 110to permit actuation thereof.

After the 0% concentration level phase of calibration is completed,block 118 generates an output to the known solvent concentration gasrelay circuitry block 120. Block 120 generates an output to theair/methane selector drive circuit in block 124 to accuate the solenoidvalve 84 in the gas analyzer to permit the known solvent concentrationgas (methane) to fill the flame cell. Another output of block 120 S isconnected to the amplifier gain potentiometer drive circuit in block 114to permit actuation thereof during this phase of calibration.

After the full scale phase of calibration is completed, signaling theend of the calibration cycle, block 120 generates an output to block 122to return solenoid 86 to its original state wherein the sample from theenclosure 10 is the input to the flame cell. A purge delay circuit inblock 126 also receives the output of block 120 and causes a one-minutedelay during which the flame cell of the gas analyzer is purged. Afterthis delay, the system automatically returns to its normal or LEL modeand calibration is completed. Outputs of block 126 control the LEL modelamp driver circuit in block 128 and the SPEED lamp driver circuit inblock 130. Outputs of block 126 are also the SPEED and SPEED signals.

Block 132 comprises circuitry which establishes the logic for FAULTconditions. One of the inputs for block 132 is generated in block 134which contains a sample low flow detector which is operably connected tolow flow switch 96 (FIG. 3) in the gas analyzer. Other inputs to block132 come from blocks 144 and 146, which contain the potentiometer limitdetectors.

Block 136 controls the propane supply to the gas analyzer. Block 138 and140 each receive an output of the bridge circuit of block 100. Block 138contains a comparator which compares the output of the bridge circuit toa preset voltage and, under the proper conditions, drives a WARNINGlamp. Block 140 contains a comparator which compares the output of thebridge circuit to a second preset voltage, and under the appropriateconditions, drives a DANGER lamp. Block 142, also connected to thebridge circuit in block 100, contains a voltage comparator which, in theevent that the flame has gone out in the gas analyzer, will drive aFLAME OUT lamp.

Variable resistors in blocks 144 and 146 are connected respectively tothe bridge calibration potentiometer MP1 and amplifier gainpotentiometer MP2 and also receive an output from block 100. Blocks 144and 146 act as limit detectors for each of the potentiometers.

Block 148 is connected to a manually actuated mode selector switch onthe front panel of the control assembly and generates the appropriatesignals to enable manual actuation of the SPEED or TEST modes. Block 150receives the output of block 126 and is utilized to initiate the"emergency stop" signal which will stop the press under a dangercondition. Block 152 represents the +17 volt power indicator. Blocks154, 156 and 158 constitute power indicators and regulators for +12volts -12 volts and +5 volts, respectively.

FIG. 5 is a detailed schematic diagram of block 100. As shown on thisdiagram, the input bridge circuit is formed of resistors 160, 162, 164,168 and 170 located in power supply 52, a resistance temperaturedetector 172, located within gas analyzer A, the bridge calibrationmotorized potentiometer MP1 and a variable resistor 174. The resistancetemperature detector 172 is preferably a platinum wire sensor whoseresistance changes as a function of temperature. Motorized potentiometerMP1 is driven, as explained in detail below, in order to calibrate thebridge to the 0% LEL setting when the zero solvent concentration gas isanalyzed, during the first phase of calibration. Variable resistor 174is utilized to trim the bridge initially.

The output of the bridge forms the input for an amplifier 176. Avariable resistor 178 is provided for manual switching into the circuit.Amplifier 176 is connected to a feedback circuit which is used to adjustthe amplifier's gain during the full scale phase of calibration. Thisgain adjustment is accomplished by means of the amplifier gain motorizedpotentiometer MP2, which is driven in the manner disclosed in detailbelow. A variable resistor 179 is used to trim the range of theamplifier. The output of amplifier 176 is limited by a diode 180. Ameter M1 is connected to amplifier 176 such that the output thereof canbe monitored. Meter M1 is protected for excessive negative voltage bydiode 182 and resistor 184. One output of amplifier 176 is fed to block102 which is schematically shown in FIG. 6.

Block 102 includes a differential amplifier 186 which is connected sothat its output will go to zero when both inputs are equal. Differentialamplifier 186 makes, by relay control, three comparisons. The first is acomparison of the set LEL voltage, derived from variable resistor 188,to the measured LEL voltage from the bridge circuit of block 100. If theoutput of the bridge circuit is higher or lower than the set LELvoltage, the appropiate signal will be generated by differentialamplifier 186 so as position the damper to bring the solventconcentration within the evaporation enclosure back to the desiredlevel. This takes place when relays 190 and 192 are in the positionsshown in FIG. 6.

During calibration and more specifically during the first phase thereof,relay 190 is actuated by block 118 to its alternate position such thatthe amplifier 186 is connected to ground. The zero solvent concentrationgas is analyzed and the output of block 100 is compared to groundthereby permitting the bridge calibration motorized potentiometer MP1 toset the bridge to the appropriate 0% LEL setting, as described below.During the second phase of calibration, relay 190 returns to itsoriginal position and relay 192 is actuated by block 120 to itsalternate position. This configuration of relays 190 and 192 causesamplifier 186 to be connected to a variable resistor 194. Resistor 194generates the known solvent concentration reference voltage to permitfull scale calibration. This voltage is compared to the output of block100 when the known solvent concentration gas is analyzed and theamplifier gain motorized potentiometer MP2 is appropriately actuated forfull scale calibration as described below. The output of differentialamplifier 186 is connected to block 108.

Block 108 contains an IC 196 (LM1414N available from NationalSemiconductor, Inc.) which acts as a zero crossing detector. The inputto IC 196 is connected to the output of differential amplifier 186 bymeans of relay 198 also actuated by block 118 to change the relay fromthe position shown to the alternate position during the first phase ofcalibration. The output of zero crossing detector 196, designated asZ_(O), is used to stop the bridge calibration motorized potentiometerMP1 from driving past the calibration point. The other output from block108, which is the LEL voltage from amplifier 186, is connected to thebridge calibration potentiometer drive circuit in block 110.

During the second phase of calibration, relay 198 returns to itsoriginal position and relay 200 is actuated by block 120 to itsalternate position, thereby connecting the output of amplifier 186 to IC202 (LM1414N available from National Semiconductor, Inc.) which acts asa zero crossing detector for full scale calibration. The output of IC202, designated as S_(O), is used to stop the amplifier gain motorizedpotentiometer MP2 from driving past the calibration point. Relay 200 isalso connected to the amplifier gain potentiometer drive circuit inblock 114.

When relays 198 and 200 are in the positions shown, the output ofamplifier 186 is connected to the input of IC 204 (5B7741393 availablefrom Fairchild) in block 104, which functions as a proportionalattenuator of the amplifier output and also as a derivative amplifierresponding to rapid changes at the input. The output from differentialamplifier 186 also forms an input to IC 206 (5B7741393 availabe fromFairchild) in block 104 which functions as an integrator for the outputof the differential amplifier. The time constants for integrator 206 areset by diode 208 and resistors 210 and 212, such that the time constantsare faster for an increasing LEL voltage than for a decreasing LELvoltage.

Also included with block 104 is an IC 214 (AHO134D available fromNational Semiconductors, Inc.) which acts as an analog switch. Analogswitch 214 has two inputs 216 and 218. Inputs 216 is connected toreceive the SPEED mode signal from block 126. When this signal ispresent, analog switch 214 inhibits the output of IC 204. Input 218 ofswitch 214 is connected to the collector of a transistor 220, the baseof which is connected to the collector of a transistor 222. The base oftransistor 220 is connected to receive the signal from tachometer 16 andif this signal is not present, i.e., the press is not operational,transistor 220 will turn on, grounding input 218 and therby inhibitingthe integral output of IC 206. Transistor 222, the base of which isconnected to receive the SPEED mode signal, will also ground the base oftransistor 220 in the presence of this signal, therby causing analogswitch 214 to inhibit the integral output of IC 206. Thus, while thesystem is operating in the SPEED mode, such as during calibration, or ifthe SPEED mode is manually enabled, the outputs of IC's 204 and 206 areinhibited. The combined outputs of 204 and 206, when same are operative,are summed at diode 224, which forms the output of block 104 and whichis connected to the input of block 106.

As seen in FIG. 7, block 106 has inputs from block 104, representing thepartially processed signal proportional to the detected solventconcentration level, from web tachometer 16 and from an exhaust damperposition potentiometer 226. The output of web tachometer 16 passesthrough a Zener barrier 228 and then through a variable resistor 230prior to forming an input to an IC 232 (5B7741393 available fromFairchild) which acts as an integrator. The output of the exhaust damperposition potentiometer 226 is connected to one of the inputs to an IC234 (5B7741393 available from Fairchild), which functions as aninverting amplifier with an adjustable offset for exhaust damperposition potentiometer compensation. The output IC 234 is then modifiedto establish two ranges of linear proportionality.

The output of IC 232 is connected to the base of a transistor 236, whichis used as a level detector to generate signal D_(L), if the feedbacksignal from the exhaust damper is invalidated. The signal D_(L) isgenerated at the collector of transistor 236. The output of IC 232 isalso connected to one of the inputs of IC 238 (5B7741393 available fromFairchild). The other input IC 238 is connected, by means of a resistor240, to the output of IC 232. IC 238 functions as proportionalattenuator and as a level shifter to properly range the output signal tothe electric to pneumatic transducer 58. Transducer 58 converts theelectrical signal output of IC 238 into a pneumatic drive which isconnected to damper position actuators 62 and 64 so as to position theexhaust and recirculation dampers.

FIG. 8 cotains schematic diagrams for block 110 and block 114, which arethe bridge calibration potentiometer drive circuit and amplifier gainpotentiometer drive circuit, respectively. During the first phase ofcalibration, the output from block 108, which represents the bridgecircuit amplifier output when the zero solvent concentration gas (air)is analyzed, is connected to one of the inputs of an IC 240 (5B7741393available from Fairchild). IC 240 functions as non-inverting amplifierwith motorized potentiometer MP1 providing a feedback signal to theother input thereof. Amplifier 240 serves to bias either transistor 242,connected to a +12 volt source, or transistor 244, connected to a -12volt source, to drive motorized potentiometer MP1 in the properdirection, that is, clockwise or counter-clockwise. A switch 246 isprovided on the front panel of the control assembly in order to manuallyactuate the motorized potentiometer. Logic gates 248, 250, 252 and 254are utilized to gate the circuit for operation. All of the inputs tothese logic gates must be high (logic one) in order to turn offtransistors 256 and 258 so as to permit MP1 to be driven. One input ofgate 248 is a signal Z_(L) which is generated by MP1 travel limitdetector of block 144. One input to gate 252 is a signal Z_(H) which isalso generated by MP1 travel limit detector of block 144. Gate 250receives logic signals Z and T, which are generated by block 118 andblock 148, respectively. The other inputs to gates 248 and 252 receivethe output of gate 250.

Block 114 has a structure quite similar to block 110. The output fromblock 112, which is representative of the output of the bridge circuitamplifier when the gas of known solvent concentration is analyzed,serves as one of the inputs of IC 256 (5B7741393 available fromFairchild). IC 256 serves as a non-inverting amplifier with motorizedpotentiometer MP2 providing a feedback signal. Amplifier 256 biasestransistor 258, connected to a +12 volt source, or transistor 260,connected to a -12 volt source, in order to drive motorizedpotentiometer MP2 in the proper direction. A switch 262 is provided onthe front panel of the control assembly in order to manually regulatethe position of motorized potentiometer MP2. Logic gates 264, 266, 268and 270 are provided and the input to each must be high or logic one, inorder to turn off transistors 272 and 274 to permit driving of thepotentiometer. The logic input to gate 264 is S_(L), one output fromblock 146. The input to gate 268 is signal S_(H) which is another outputof block 146. The inputs to gate 266 are S and T, generated by block 120and block 148, respectively.

FIG. 9 contains schematic diagrams of the warning level comparator anddriver circuit of block 138, danger level comparator and drive circuitof block 140 and "flame out" driver circuit of block 142. The input toblocks 138 and 140 is connected to block 100 and more particularly, tothe output of amplifier 176 where this output is connected to the modebetween diodes 180 and 182. Thus, the input to blocks 138 and 140represents the amplified bridge circuit output. Block 138 contains an IC276 (LM1414N available from National Semiconductors, Inc.) which isutilized as a voltage comparator. The comparison voltage is set by avariable resistor 278, which represents the warning level, that is, thesolvent concentration level above which the WARNING lamp 280 is to beenergized. The output of the voltage comparator drives WARNING lamp 280through transistor 282 and 284. The base of transistor 286 is connectedto receive the CAL signal output of block 116. The presence of the CALsignal causes transistor 286 to turn on, thereby grounding the base oftransistor 284 and preventing the actuation of WARNING lamp 280 duringthe calibration cycle.

Block 140 is structurally similar to block 138. This block contains anIC 288 (LM1414N available from National Semiconductor, Inc.), one inputof which is connected to receive the amplified bridge output signal. Theother input of IC 288 is connected to a variable resistor 290, whichsets the danger level voltage. IC 288 acts as a voltage comparator andwhen a voltage above the level set by resistor 290 is sensed,transistors 292 and 294 are turned on thereby generating the DANG signaland actuating DANGER lamp 296. The base of the transistor 298 isconnected to receive the CAL signal output from block 116 therebyinhibiting the actuation of DANGER lamp 296 during the calibrationcycle.

The input of block 142 is connected to the output of bridge circuit inblock 100 at the output of amplifier 176. This input is connected to thebase of a transistor 300, which is utilized as a voltage level detector.When an output signal from bridge circuit 100 is detected to be under acertain magnitude, the FLAME OUT lamp 302 is turned on to indicate thatthe flame has gone out in the gas analyzer. Logic signal F is alsogenerated by this circuit representing the flame out condition.

FIG. 10 shows a schematic representation of the mode switch 304, SPEEDmode enable circuit of block 148, LEL mode lamp driver circuit of block128, power indicator circuit of block 152, power circuit and regulatorcircuit of block 154 and power indicator and regulator circuit of block156. Mode switch 304 has four positions, off, LEL mode, SPEED mode andTEST mode. FIG. 10 shows mode switch 304 in the off position. Whenswitch 304 is in the speed mode position, the SPEED signal is generatedas an output of block 148 at mode 306. When mode switch 304 is in thetest mode position, block 148 generates two complimentary logic signalsT and T. The output upon which T is generated is connected to thecollector of a transistor 308, that base of which is connected to themode switch 304.

Block 128 contains the LEL mode lamp driver circuit. This circuitconsists of two transistors 310 and 312. The base of transistor 310receives the complement of the SPEED signal and when this signal ispresent, that is, when the system is not in the speed mode, LEL lamp 314is energized.

Blocks 152, 154 and 146 each contain a light emitting diode 316, 218 and320, respectively, which act as an indicator to show the presence of +17volt, +12 volt and -12 volt power voltages respectively. Block 154 alsocontains a voltage regulation circuit comprised of diode 322 andcapacitor 324. Likewise, block 146 contains a diode 326 and a capacitor328 which act to regulate the -12 volt output and diodes 330 and 332which act to regulate the -6 volt output.

FIG. 11 shows a schematic diagram of the calibration trigger circuit ofblock 116 and zero solvent concentration gas relay control circuit ofblock 118. Block 116 contains an IC 334 (LM1414N available from NationalSemiconductor, Inc.) and transistors 336 and 388, which along withassociated components function as an astable multi-vibrator with afrequency of about 22 minutes. The output of the multi-vibrator isconnected to the base of transistor 388 which triggers an IC 340(SN74L93 availabe from Texas Instruments) which functions as a divide by16 counter, thereby generating an output approximately every six hoursto initial calibration. The calibration trigger signal CAL apperars atthe collector of transistor 344.

The calibration trigger signal CAL forms the input to the zero solventconcentration gas relay control circuit of block 118. Block 118 is thefirst stage to respond to the calibration trigger. The CAL signal inputis fed to the base of a transistor 346 which in turn operates transistor348 and thus, relay 350 which in turn operates switches 190 and 198 (seeFIG. 6) to commence the first phase of calibration. This will occur aslong as the FAULT signal (from block 132) is not present as an input totransistor 384. A manual calibration initiation switch 354 is providedand when actuated will trigger the calibration cycle as long as theSPEED mode signal is present at the base of transistor 356.

IC 358 (5B7741393 available from Fairchild), one input of which isconnected to the output circuit of transistor 348, acts as anintegrator. When the output of IC 358 reaches a positive voltage,transistor 360 turns on, latching the bistable circuit formed of gates362 and 364 to generate the Z and Z logic signals, repsectively, theoutput of gate 362 is connected to the base of a transistor 366. Timingcapacitor 368, connected across the output circuit of transistor 366,discharges through transistor 366 to reset the bistable circuit.Transistor 370, whose base is connected to receive the S signal, throughdiode 352 and the FAULT signal through diode 353, also acts as a resetfor the bistable circuit comprised of gate 362 and 364. The S signaloutput from block 120 represents the actuation of the second phase ofcalibration. When the bistable circuit resets, relay 350 is de-energizedand the second phase of the calibration cycle is initiated. It should beappreciated that the output of gate 364, Z is connected as an input tobridge calibration potentiometer drive circuit in block 110 and it isduring the time when the circuit is latched that the potentiometer MP1will be driven.

FIG. 12 shows schematic diagrams of the known solvent concentration gasrelay control circuit of block 120 and propane driver and time circuitof block 136. When the signal ouput Z of gate 362 (FIG. 11) is high orlogic one, the bridge calibration potentiometer MP1 will be driven untilthe output Z_(O) of the zero crossing detector circuit of block 108 goeshigh or logic one. Signals Z and Z_(O) constitute the inputs to a gate372 hich in turn feeds gate 374. The output of gate 374 is connected tothe base of transistor 376. When actuated, transistor 376 turns ontransistor 378 which in turn energizes relay 380 to actuate switches 192and 200 (FIG. 6) connecting the system for full scale calibration. Thisoccurs as long as the FAULT signal (from block 132) is not present online 382. Turning on a relay 380 causes the generation of the logic Ssignal, this causing relay 350 (FIG. 11) to be deactuated.

IC 388 (5B7741393 available from Fairchild), one input of which isconnected to the output circuit of transistor 378, acts as anintegrator. When the output of IC 388 reaches a positive voltage,trnsistor 390 is turned on thereby latching the bistable circuitcomprised of gates 392 and 394, to generate the S and S logic signals.Transistor 396 acts as a reset for this latch, the base thereof beingconnected to an output of block 118.

Timing capacitator 398 discharges through the output circuit oftransistor 400 when the S logic signal, connected to the base thereof,is high or in the logic one state to reset the bistable circuit. The Ssignal provides one of the inputs to gate 402. The other input to gate402 is the S_(O) signal, generated by zero crossing detector circuit ofblock 112. The output of gate 402 provides one of the inputs for gate404, the other being the FAULT signal received from block 132. Whensignal S is high or logic one, the amplifier gain potentiometer drivecircuit of block 114, one of whose inputs is the S signal, will drivemotorized potentiometer MP2 until the output S_(O) of the zero crossingdetector circuit of block 112 goes high or logic one. When this occurs,transistor 406 causes relay 380 to turn off, thereby causing switches192 and 200 (FIG. 6) to return to their original positions.

Also shown in FIG. 12 is a schematic diagram of the propane driver andtimer circuit of block 136. This circuit comprises an IC 408 (5B7741393available from Fairchild) which operates as an integrator, holdingtransistor 410 in its "on" condition for 14 minutes after ignitionswitch 412 is depressed. The base of transistor 410 is also connected toreceive the complement of the FLAME OUT signal F and transistor 410 willremain on as long as this signal is present. Transistor 410 controls thepropane supply to the gas analyzer.

FIG. 13 shows a schematic diagram of the power indicator and regulatorcircuit of block 158 and a schematic diagram of the MP1 travel limitdetector circuit of block 144. Block 158 is connected to the powersupply and contains a light emitting diode 414 which indicates thepresence of the +5 voltage from the power supply. Voltage regulation isaccomplished by means of resistor 416, diode 418 and capacitator 420.

Block 144 is the MP1 travel limit detector circuit. Potentiometer 422 ismechanically connected to the shaft of motorized potentiometer MP1.Transistors 424 and 426 and gates 428 and 430 form an electric limit onthe travel of motorized potentiometer MPL with logic outputs Z_(L) andA_(H) to the bridge calibration potentiometer driver circuit of block110 and an output from diode 432 to the fault indicator circuit of block132.

FIG. 14 shows a schematic diagram of the MP2 travel limit detectorcircuit of block 146, fault indicator circuit of block 132 and samplelow flow detector circuit of block 134. Circuit 146 is essentially thesame as circuit 144. It contains a potentiometer 434 which is connectedto the shaft of motorized potentiometer MP2. Transistors 436 and 438,along with gates 440 and 442, form an electronic limit on the travel ofamplifier gain potentiometer MP2 and provide logic inputs S_(L) andS_(H) to the amplifier gain potentiometer drive circuit of block 114 andthe FAULT indicator circuit of block 132.

The output from gate 430 of block 144 and of gate 442 in block 146 arecombined at node 444 as an input to fault indicator circuit of block132. Node 144 is connected to the base of transistor 446 which in turndrives transistor 448. Transistors 446 and 448 establish the logic for aCAL RED condition. The output circuit of trnsistor of 448 is connectedto the base of transistor 450 which acts as a driver for the CAL REDlamp 452, located on the front panel of the component. Transistors 454and 456 establish the logic for the fault condition and the output takenat the collector of transistor 456 constitutes the FAULT signal and isconnected to block 118 among others. The output at the collector oftransistor 454 is connected to circuit 120.

Block 134 has connected thereto, as an input, low flow switch 96situated within the gas analyzer. This switch is connected to the baseof transistor 458. The output circuit of transistor 458 causes thegeneration of a FAULT signal through connection with block 132 by meansof line 460, which is connected to the base of transistor 454. Thus, aFAULT signal is generated when the flow within the gas analyzer is belowa given value. The output circuit of transistor 458 is also connected tothe base of a transistor 462 which acts as a driver for LOW FLOW lamp464, which is present on the front panel of the control assembly.

FIG. 15 shows schematic diagrams of the emergency stop logic circuit 150and speed lamp driver circuit 130. Block 150 receives the CAL signal(from block 116) at one of its inputs and this input is connected to thebase of transistor 466, the collector of which is connected as an inputto gate 468. The other input to gate 468 is the DANG signal from block140. The output of gate 468 is connected to the input of gate 470. Theother input of gate 470 is the output of a gate 472, the inputs of whichare the D_(L) and SPEED signals from blocks 106 and 126, respectively.Gates 468, 470 and 472 will cause transistor 474, the base of which isconnected to the output of gate 470, and transistor 474, the base ofwhich is connected to the collector of transistor 472, to latch togenerate an "emergency stop" trigger signal at node 476, which isconnected to the power supply assembly. When the danger signal ispresent during the LEL or SPEED mode functions or the D_(L) signal ispresent when in the SPEED, TEST or CAL modes, the "emergency stop"signal is generated.

Block 130 contains the SPEED lamp driver circuit. This circuit has aninput the SPEED signal, which is connected to the base of a transistor478. The emittor of transistor 478 is connected to the base oftransistor 480, which serves a driver for SPEED lamp 482.

FIG. 16 contains schematic diagrams of the purge delay circuit of block126; test/sample selector drive circuit of block 122 and air/methaneselector drive circuit of block 124. Block 126 receives inputs fromblocks 118 and 120, which are connected to the base of transistor 480.Transistor 480 acts as the CAL GREEN lamp 482 driver. The collector oftransistor 480 is connected to the base of a transistor 482, thecollector of which is connected to one input of IC 484 (5B7741393available from Fairchild). IC 484 acts as an integrator, generating aone minute delay to permit purging of the flame cell until transistor486, the base of which is connected to the output of IC 484, turns off.When transistor 486 turns off, the SPEED signal is generated as anoutput thereof. The SPEED signal causes the system to return the LELmode. At this point the calibration cycle is complete. The collector oftransistor 486 is connected to a gate 488, the output of which isconnected to one of the inputs of transistors 472 in block 150. A secondinput of this block is connected to the gate of transistor 478 of block130.

Block 122, which is the test/sample selector driver circuit, receives aninput from block 118 and a second input from block 120, which arecombined and fed to the base of transistors 490. Transistors 490 drivessolenoid 86 of gas analyzer A which selects the test or sample input tothe flame cell.

Block 124 which is the air/methane selector drive circuit has an inputfrom block 120 which is connected to the base of a transistor 492. Thebase of transistor 492 is also connected to a manual air/methaneselector switch 494. Transistor 492 acts as a driver for solenoid 84which selects the air or methane inputs to the gas analyzer flame cell.

It will therefore be appreciated, that the present invention relates toan LEL control which, under normal conditions, controls the position ofan exhaust damper in accordance with the sensed solvent concentrationlevel within the evaporation enclosure and the web speed. The systemincludes means for automatically, periodically calibrating theresistance bridge and amplifier therefor, during which time the damperposition is controlled in accordance with the speed of the web alone.Calibration of the resistance bridge and amplifier takes place in twophases: first, zero solvent concentration gas is fed to the gas analyzerand the bridge circuit is calibrated at zero solvent concentration levelby means of a motorized potentiometer; second, a gas of a known solventlevel concentration is analyzed in the gas analyzer and the gain of theamplifier is calibrated in accordance with the known solventconcentration of the gas. Thus, the system is calibrated at both thezero and full scale levels. Thereafter, the system returns to its normalmode of operation. The system also includes means for continuouslymonitoring the solvent concentration level and for generating an"emergency stop" signal, to stop the press, in the event that thesolvent level concentration within the evaporation enclosure reaches adangerous level.

While only a single preferred embodiment of the present invention hasbeen disclosed herein for purposes of illustration, it is obvious thatmany moidifications and variations could be made thereto. It is intendedto cover all of these variations and modifications which fall within thescope of the invention as defined by the following claims.

I claim:
 1. An LEL control for regulating the exhaust from anevaporation enclosure through which a solvent laden web passescomprising means operably connected to the enclosure for sensing thesolvent concentration therein and for generating a first signaldependent thereupon, means operably associated with the web for sensingthe speed thereof and for generating a second signal dependentthereupon, means for combining said first and said second signals toform a control signal, means for regulating said exhaust in accordancewith said control signal, and means for calibrating said first signalgenerating means, said calibrating means comprising means for inhibitingthe output of said first signal generating means during calibration. 2.The control of Claim 1 wherein said calibration means comprises meansfor calibrating said first signal generating means at the zero settingand means for calibrating said first signal generating means at the fullscale setting.
 3. The control of claim 2 wherein said calibration meansfurther comprises means for actuating said zero setting calibrationmeans for a first given time interval and means for actuating said fullscale setting calibration means for a second given time interval.
 4. Thecontrol of claim 3 wherein said calibration means further comprisesmeans for periodically generating a calibration initiation signal, saidzero setting calibration actuation means being actuated by saidinitiation signal.
 5. The control of claim 4 wherein said full scalesetting calibration actuation means is actuated after said first giventime interval.
 6. The control of claim 5 wherein said calibration meansis deactuated after said second time interval.
 7. The control of claim 2wherein said calibration means further comprises means for periodicallygenerating a calibration initiation signal, said zero settingcalibration means being actuated by said initiation signal.
 8. Thecontrol of claim 1 wherein said first signal generating means comprisesa gas analyzer and a bridge circuit, said bridge circuit comprising avariable resistance means for sensing the temperature in said analyzer,and amplification means for amplifying the output of said bridgecircuit.
 9. The control of claim 8 wherein said first signal generatingmeans further comprises means for processing the output of saidamplification means, said processing means comprising means forsupplying a plurality of reference voltages, means for selecting one ofsaid reference voltages and means for comparing said amplifier outputwith said selected reference voltage and producing an output inaccordance with said comparison.
 10. The control of claim 9 wherein saidcombining means comprises means for combining said comparison meansoutput with said second signal to form said control signal.
 11. Thecontrol of claim 2 wherein said first signal generating means comprisesa gas analyzer and a bridge circuit, said bridge circuit comprising avariable resistance means for sensing the temperature in said analyzer,and amplification means for amplifying the output of said bridgecircuit.
 12. The control of claim 11 wherein said first signalgenerating means further comprises means for processing the output ofsaid amplification means, said processing means comprising means forsupplying a plurality of reference voltages, means for selecting one ofsaid reference voltages and means for comparing said amplifier outputwith said selected reference voltage and producing an output inaccordance with said comparison.
 13. The control of claim 12 whereinsaid combining means comprises means for combining said comparison meansoutput with said second signal to form said control signal.
 14. Thecontrol of claim 11 wherein said zero setting calibration meanscomprises means for adjusting said bridge to the zero setting.
 15. Thecontrol of claim 11 wherein said full scale calibration means comprisesmeans for adjusting the gain of said amplification means to the fullscale setting.
 16. The control of claim 14 wherein said full scalecalibration means comprises means for adjusting the gain of saidamplification means to the full scale setting.
 17. The control of claim14 wherein said bridge circuit further comprises a potentiometer andwherein said zero setting calibrating means comprises means foradjusting said potentiometer to calibrate said bridge circuit to thedesired zero setting.
 18. The control of claim 1 wherein saidcalibration means further comprises means for periodically initiatingcalibration.
 19. The control of claim 18 wherein said calibrationinitiation means comprises timing means for generating a calibrationinitiation signal at preset intervals.
 20. The control of claim 1further comprising means for generating an emergency stop signal whensaid first signal exceeds a preset level.
 21. The control of claim 1further comprising manually actuated means for inhibiting the output ofsaid first signal generating means.
 22. The control of claim 1 whereinsaid second signal generating means comprises a tachometer operablyassociated with said web.
 23. The control of claim 1 wherein said firstgenerating means comprises a gas analyzer, a first source of solventfree gas, a second source of known solvent concentration gas, valvemeans normally conditioned to connect said analyzer to said enclosure,and wherein said calibration means further comprises means forconditioning said valve means to connect said analyzer to said firstsource, during a zero setting phase of calibration and to said secondsource, during a full scale setting phase of calibration.
 24. Thecontrol of claim 23 wherein said first signal generating means furthercomprises a bridge circuit said bridge circuit comprising a variableresistance means for sensing the temperature in said analyzer and apotentiometer, and means for amplifying the output of said bridgecircuit.
 25. The control of claim 1 wherein said first signal generatingmeans comprises means for generating a signal which varies with thesensed solvent concentration, means for supplying a plurality ofreference voltages, means for selecting one of said reference voltages,and means for comprising said preliminary signal and said selectedreference voltage and for producing an output in accordance with saidcomparison.
 26. The control of claim 25 wherein said combining meanscomprises means for combining said comparison means output with saidsecond signal to form said control signal.
 27. The control of claim 25wherein said calibration means comprises means for calibrating saidfirst signal generating means at the zero setting and means forcalibrating said first signal generating means at the full scalesetting.
 28. The control of claim 27 wherein said zero settingcalibraton means comprises means for adjusting said bridge to the zerosetting.
 29. The control of claim 27 wherein said full scale calibrationmeans comprises means for adjusting the gain of said amplification meansto the full scale setting.
 30. The control of claim 25 wherein saidreference voltages comprise a preset LEL reference voltage, a zerosetting reference voltage and a full scale reference voltage.
 31. Thecontrol of claim 30 wherein said calibration means comprises means forperiodically initiating calibration.
 32. The control of claim 31 whereinsaid calibrating means further comprises means, effective whencalibration is initiated, for conditioning said reference voltage, suchthat said comparison means output represents the zero settingcalibration signal.
 33. The control of claim 32 wherein said calibrationmeans further comprises timing means for generating a timing signalafter a given time interval to initiate the full scale phase ofcalibration.
 34. The control of claim 33 wherein said calibration meansfurther comprises means, actuated upon receipt of said timing signal,for conditioning said reference voltage selecting means to select saidfull scale reference voltage, such that the output of said comparisonmeans represents the full scale calibration signal.
 35. The control ofclaim 34 wherein said calibration means further comprises means foradjusting the gain of said amplification means in accordance with saidfull scale calibration signal.
 36. The control of claim 35 wherein saidcalibration means further comprises second timing means for generating asecond timing signal after a given time interval to terminatecalibration.
 37. The control of claim 35 wherein said calibration meansfurther comprises means, actuated upon receipt of said second timingsignal, to actuate said reference voltage selecting means to select saidpreset LEL reference voltage and to deactuate said output inhibitingmeans.
 38. The control of claim 25 wherein said first generating meanscomprises a gas analyzer, a first source of solvent free gas, a secondsource of known solvent concentration gas, valve means normallyconditioned to connect said analyzer to said enclosure, and wherein saidcalibration means further comprises means for conditioning said valvemeans to connect said analyzer to said first source, during a zerosetting phase of calibration and to said second source during a fullscale setting phase of calibration.
 39. The control of claim 38 whereinsaid reference voltages comprise a LEL reference voltage, a zero settingreference voltage and a full scale reference voltage.
 40. The control ofclaim 39 wherein said calibration means comprises means, effective whencalibration is initiated, to condition said valve means to select saidfirst source and to condition said reference voltage selecting means toselect said zero setting reference voltage such that said comparisonmeans output represents the zero setting calibration signal.
 41. Thecontrol of claim 40 wherein said calibration means further comprisestiming means for generating a timing signal after a given time intervalto initiate the full scale phase of calibration.
 42. The control ofclaim 41 wherein said calibrating means further comprises means,actuated upon receipt of said timing signal, for conditioning said valvemeans to select said second source and for conditioning said referencevoltage selecting means to select said full scale reference voltage suchthat the output of said comparison means represents the full scalecalibration signal.
 43. The control of claim 42 wherein said calibratonmeans further comprises means for adjusting the gain of saidamplification means in accordance with said full scale calibrationsignal.
 44. The control of claim 42 wherein said calibration meansfurther comprises second timing means for generating a second timingsignal after a given time interval to terminate calibration.
 45. Thecontrol of claim 44 wherein said calibration means further comprisesmeans actuated upon receipt of said second timing signal, to actuatesaid reference voltage selecting means to select said LEL referencevoltage and to deactuate said output inhibiting means.